This invention relates generally to the field of imaging, and more particularly to a method and device for calibrating an imaging apparatus having multiple imaging outputs.
Many existing printing devices are bi-level devices that cannot readily reproduce continuous tone images. In these devices, a continuous tone image is approximated by first defining a halftone grid, known as a screen. The screen is essentially an array of regions known as halftone cells. Each halftone cell typically has a fixed size, and is defined by a matrix of addressable pixels that can be selectively turned xe2x80x9conxe2x80x9d in a digital, xe2x80x9cbi-levelxe2x80x9d manner to form various patterns. The human eye integrates the array of halftone cells to form a visual perception of a continuous tone image. A gray value is assigned to each halftone cell within the screen in order to represent the gray value of the corresponding areas of the continuous tone image. By activating a percentage of the pixels contained within each halftone cell, the cell simulates a shade of gray that closely approximates the respective area of the continuous tone image. For example, in order to approximate a lighter area of the image, a smaller percentage of pixels, such as 10%, of the halftone cell will be activated. To simulate a darker image region, a higher percentage of the pixels will be activated.
A conventional printing device produces halftone images by forming halftone dots on a medium at locations corresponding to each pixel that has been turned xe2x80x9conxe2x80x9d in the respective halftone cell. The process of forming the halftone dot is particular to the type of printing device. For example, the spots may be formed by depositing ink or toner on a printing substrate at locations corresponding to the activated points. Alternatively, spots may be formed on a photographic film or a thermographic film by exposure to a radiation or thermal energy, respectively. Other printing devices employ processes such as dye sublimation or thermal mass transfer as are known in the art.
Many printing devices reproduce an original color image by separating the image into color components such as yellow, cyan, magenta and black. The color components are independently formed on a respective medium according to the halftone process described above. For example, in offset printing a printing plate is created for each color component and the color image is reproduced by overprinting colored inks.
Dot gain is a well known problem associated with halftone systems and refers to an apparent change in size of a printed halftone dot from its target size. This phenomenon is caused by many factors such as a tendency of ink to spread or variations in film characteristics. For example, when 50% of the dots within a halftone cell are exposed, the resulting dark area may cover more than or less than 50% of the total area defined by the halftone cell. Typically, this is due to nonlinear effects in the imaging system, film, media or processing system. Because 0% and 100% are usually achievable, a non-linear relationship may exist between the target dot area and the resultant dot area.
A subset of conventional printing devices, referred to as imagesetters, consist of a front-end raster image processor (RIP) and a recording device for producing the image on film, paper or a printing plate. Manual calibration techniques are well known in the industry as a means for calibrating a halftone imagesetter so as to compensate for dot gain. Typically an operator of a printing device uses a densitometer to detect dot gain. A densitometer is an instrument that measures the perceived optical density of the reproduced image. A densitometer typically consists of a light emitting component for illuminating the reproduced image and a photodetector for measuring light reflected from the image. Alternatively, the photodetector measures light transmitted through the reproduced image. The darker the image the more light it absorbs and the higher the density reading from the densitometer. During the calibration process a single grayscale test pattern is printed which includes a series of halftone image regions. Each image region has a different predetermined dot area. For example a series of image regions is usually printed such that the dot areas range from 2% to 100%. The operator manually measures the density of each image region with a standard densitometer. From these measurements, a xe2x80x9ctransfer finctionxe2x80x9d is created to map any subsequently requested dot area to a dot area which produces the correct visual density.
Conventional calibration methods operate at either the application level or at the RIP level. Application level methods send a transfer function with each print job. On the other hand, RIP-based compensation techniques require the RIP to store transfer functions. The operator selects the correct transfer function based on current operating conditions. If the operating conditions change, such as the use of a new media type, the operator generates a grayscale test pattern, manually measures the densities with a densitometer, generates a transfer ftnction and designates the new function for current use. If no major system change occurs, the functions may be used for an extended period such as several weeks.
The above-described calibration approaches require the operator to determine when calibration is appropriate and therefore require substantial operator interaction. Additionally, such manual approaches are inadequate in imaging systems having multiple imaging outputs such as several laser diodes. Manual calibration of such systems is too time consuming and yield results that are only marginally acceptable.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method and device for calibrating an imaging system having multiple imaging outputs without requiring operator intervention.
To overcome the problems in the prior art, the present invention provides an improved method and device for calibrating an imaging apparatus that has multiple imaging outputs such as a plurality of laser diodes. Calibration is achieved by driving each imaging output to form a test pattern having at least one image region on an imaging element. Each image region has a target optical property such as a target optical density, a target sharpness, etc. Next, an optical property, such as optical density, is measured for each formed image region. The imaging outputs are adjusted as a function of the measured optical properties, thereby reducing optical variations between the regions that are imaged by the multiple imaging outputs.
More specifically, the imaging outputs are adjusted by characterizing the measured optical property as a function of one or more imaging variables. A response curve is generated for each set of image regions that have substantially equal target optical properties. For example, if each imaging output forms a grayscale, a response curve is generated for the image regions having substantially equal target densities. The imaging variables for each imaging region are adjusted by reducing a difference between the measured optical property and the response curve. For example, in one embodiment the imaging outputs are a plurality of pulsed laser diodes such that the adjusted imaging variables include a pulse duration and a laser current. These and other features and advantages of the invention will become apparent from the following description of the preferred embodiments of the invention.